1. Field of the Invention
This invention relates to an improved process for regenerating carrier catalysts used in the preparation of ethylene oxide. The carrier catalysts primarily consist of silver and, in some cases, alkali metal compounds, in particular cesium, as promoters.
2. Description of the Prior Art
Carrier catalysts as well as their preparation are generally known, for example, in published document EP-B1 00 14 457 (assigned to BASF AG and hereinafter referred to as EP-B1). In and of themselves carrier catalysts are very suitable for the preparation of ethylene oxide from ethylene and oxygen. However, since their effectiveness diminishes the longer they are in service, efforts have been made to find ways to regenerate them without having to remove them from the ethylene oxide reactor.
Thus, various treatment processes have been developed in which the deactivated catalyst is treated with compounds of heavy alkali metals, in particular with cesium compounds. In published documents DE-A 25 19 599 and 26 11 856 (Hoechst AG) the deactivated catalyst is treated with alcohol solutions of the alkali metal compounds; in published documents DE-A 26 49 359 (Hoechst AG) this treatment is performed in at least two steps, whereby the ethylene oxide production is continued between these steps; in published document DE-A 27 40 480 (Hoechst AG) the catalyst is washed in an inert liquid prior to treatment with the alkali metal; in DE-A 29 38 245 (Hoechst AG) the washing fluid contains ammonia or aliphatic amines; and in EP-B1, cited previously, the catalyst is treated in the presence of a surfactant and, preferably, a complexing agent such as 2-aminobutane.
EP-B1, in addition to disclosing the regeneration of the catalyst, also teaches its preparation and thus also the initial treatment of the fresh silver catalyst with the compounds of the heavy alkali metals and a reducing agent such as hydrazine, optionally with a surfactant as well as a complexing agent for silver (I) ions. However, EP-B1 does not teach the use of a reducing agent in the regeneration of the catalyst.